1. Field of the Invention
The present invention relates to a power amplifier module for a portable terminal used in a mobile communication system, particularly to a power amplifier module for a cellular telephone system requesting a high tolerance level of device destruction against significant load impedance fluctuation.
2. Description of the Related Art
In recent years, expansion of a mobile communication market represented by a cellular telephone system is remarkable and higher efficiency formation of a portable terminal has been requested in aiming at enlargement of speech time period. In order to realize the request, among parts constituting a portable terminal, it is indispensable to achieve low power consumption formation, that is, high efficiency formation of a power amplifier requesting the largest power consumption. Meanwhile, when a portable terminal is used, there frequently causes destruction of an antenna constituting a load of the power amplifier module or contact of antenna with metal. In such an occasion, a condition of matching the power amplifier module and the antenna is destructed and therefore, a large standing wave is formed by power reflection and the power amplifier module is liable to destruct. Therefore, a large tolerance level of device destruction is requested for an amplifying device constituting the power amplifier. Although currently, as an amplifying device of a power amplifier, Si-MOSFET having a large tolerance level of device destruction is mainly reduced into practice, in recent years, development of a power amplifier constituting an amplifying device by GaAs-HBT (Heterojunction Bipolar Transistor) capable of achieving high efficiency more than Si-MOSFET has actively been promoted. However, a tolerance level of device destruction of GaAs-HBT is smaller than that of Si-MOSFET and for reduction into practice, protection against device destruction in load impedance fluctuation becomes an indispensable problem. Conventionally, according to a power amplifier using an Si transistor, as disclosed in Japanese Patent Laid-Open No. 135809/1990, in order to avoid destruction of an amplifying device caused by shortcircuit of load or the like, there have been carried out many trials for inserting a circuit for detecting overcurrent on the emitter side or the collector side of a power amplifying element.
However, the method of inserting the circuit for detecting overcurrent to the emitter side or the collector side of the power amplifying element, causes power loss and gives rise to lowering of efficiency and therefore, the method is not suitable for a power amplifier for a cellular telephone system requesting high efficiency. Meanwhile, there is hardly seen an example of protection against device destruction of a power amplifier using GaAs-HBT as an amplifying device. As a representative example, there is conceived a method of supplying base current of GaAs-HBT from a current source and setting the base current to reduce when power supply voltage is elevated to thereby prevent destruction by restraining rise of output power.
FIG. 3A shows a final stage amplifier portion of the conventional example. Numerals 1 and 2 respectively designate an input and an output terminal, numeral 3 designates GaAs-HBT, numerals 4 and 5 designate matching circuits, numerals 6 and 7 designate bias resistors, numeral 8 designates a high frequency choke inductor, numeral 9 designates an output control terminal, numeral 10 designates a current source, numeral 11 designates a power supply voltage detecting circuit and numeral 12 designates a power supply voltage terminal. In FIG. 3A, an operating point of GaAs-HBT3 is determined by idling current (collector current when there is not a signal). Therefore, in order to set the operating point, base current in a unique relationship with the idling current may be set to a required value. From such reason, according to the conventional example of FIG. 3A, there is constructed a constitution of supplying the base current from the current source 10. Further, when output control voltage exceeds a predetermined value, the power supply voltage detecting circuit 11 is operated and the idling current is maintained constant by restricting the output control voltage applied to the current source 10. The value of the idling current is set in accordance with power supply voltage and the higher the power supply voltage, the smaller the idling current. FIG. 3B shows a relationship between output power and power supply voltage when the output control voltage in the above-described conventional example is 2.2V. It has been found by experiment by the inventors that the output voltage shows a reducing tendency when the power source voltage is equal to or higher than 3.7 through 4V. This reflects a result that the idling current is controlled to reduce in accordance with rise of the power supply voltage.
Further, according to the above-described conventional example, as shown by FIG. 3B, temperature dependency of the output voltage is significant. This is because the base current of GaAs-HBT3 is supplied from the current source, whereby not only the current amplification rate of GaAs-HBT3 and temperature variation of the current source but also production deviation are liable to be influenced thereby and it is difficult to realize high product yield.
It is an object of the invention to resolve the above-described problem of the conventional technology and provide a power amplifier module at a low cost having high efficiency and high tolerance level of device destruction and strong at production deviation and temperature variation of GaAs-HBT and the base current supply source.
It is other object of the invention to protect a power amplifier module against destruction with regard to a wide range of phase change in load variation by detecting, canceling or restricting overcurrent flowing at the base of the final stage amplifier GaAs-HBT produced in load variation.
It is other object of the invention to achieve further promotion of a tolerance level of device destruction of GaAs-HBT by also using a function of successively lowering idling current in accordance with power source voltage or clipping function of a diode in addition to a constitution of detecting, canceling or restricting the overcurrent.
It is other object of the invention to be able to carry out stable high-quality signal transmitting operation in a mobile communicator even when a user erroneously destructs an antenna or the like or brings the antenna into contact with a metal or the like.
It is other object of the invention to provide a power amplifier module having insignificant influence of the current amplification rate of GaAs-HBT on the production deviation or the temperature variation, providing high production yield and capable of contributing to low cost formation.
The above-described and other objects and novel characteristics of the invention will become apparent from the description and the attached drawings of the specification.
An explanation will be given of outlines of representative aspects of the invention disclosed in the application as follows.
That is, in order to achieve the above-described object, the invention provides means in which required output power can be ensured when a power amplifier is operated in steady state (for example, 50xcexa9 load), further, when load variation or power supply voltage rise is caused, in order to prevent base current in a bipolar transistor constituting a power amplifier element from flowing more than necessary, in the case in which an amount of overcurrent is produced by exceeding a previously set current value, the amount of overcurrent is detected and is negatively fed back to base current (idling current) supplied to the power amplifier element and the amount of overcurrent is subtracted from initial idling current to thereby prevent destruction of the power amplifier element while ensuring required output power. Here, although as the bipolar transistor constituting the power amplifier element, GaAs-HBT can be used, the bipolar transistor is not limited thereto but other kind of bipolar transistor such as SiGe-HBT or the like may be used.
When the power amplifier is operated in a steady state, a set value for detecting the amount of overcurrent is set to a value slightly higher than the base current necessary for providing required output current. This is for preventing a protecting circuit from being brought into an operating state erroneously in the steady state. For such purpose, the protecting circuit for detecting the amount of overcurrent and negatively feeding the amount of overcurrent back to the idling current, is brought into a standstill state and when the load variation or the power supply voltage rise is caused, the current is increased and when the current exceeds the set value, the protecting current starts operating and the excess amount of the base current is canceled. A voltage source is used for supplying the base current of the final stage amplifier GaAs-HBT.
Although it is preferable to use a bipolar transistor for a transistor constituting the protecting circuit, the transistor is not limited thereto but Si-MOSFET may also be used.